Gas Exchange Systems and Methods for Calculating Ventilatory Threshold and Evaluating Pulmonary Arterial Hyertension

1. A gas exchange system, the system comprising: a flow sensor configured to sense a respiratory flow of a patient; an analyzer configured to determine a composition of at least a portion of the respiratory flow of the patient; and a computing device configured to: receive breath-by-breath gas exchange measurements based on the respiratory flow of the patient captured by the flow sensor and the analyzer during a submaximal gas exchange test; group the breath-by-breath gas exchange measurements into multi-breath groups, each multi-breath group containing measurements for a plurality of sequentially occurring breaths; generate a multi-breath average minute ventilation for each of the multi-breath groups by averaging minute ventilation values for each of the breaths therein; generate a multi-breath average rate of oxygen uptake for each of the multi-breath groups by averaging rate of oxygen uptake values for each of the breaths therein; plot a data point for each of the multi-breath groups based on the multi-breath average minute ventilation and the multi-breath average rate of oxygen uptake; and estimate a ventilatory threshold based on the plot.

2. The gas exchange system of claim 1 , wherein the computing device is further configured to: calculate parameter scores based upon an end-tidal partial pressure of carbon dioxide (PetCO2) value at the ventilatory threshold and a minute ventilation/carbon dioxide production relationship (VE/VCO2) value at the ventilatory threshold; estimate a likelihood that the patient has pulmonary arterial hypertension based on the parameter scores; and cause a user interface to be displayed, the user interface including a report that shows the estimated likelihood.

3. The gas exchange system of claim 2 , wherein the computing device being configured to calculate the parameter scores includes the computing device being configured to assign a PetCO2 parameter score based on a result of comparing the PetCO2 value at the ventilatory threshold to at least one predetermined threshold value for PetCO2.

4. The gas exchange system of claim 3 , wherein the computing device being configured to calculate the parameter scores includes the computing device being configured to assign a VE/VCO2 parameter score based on the result of comparing the VE/VCO2 value at the ventilatory threshold to at least one predetermined threshold value for VE/VCO2.

5. The gas exchange system of claim 4 , wherein the computing device being configured to estimate the likelihood that the patient has pulmonary arterial hypertension includes the computing device being configured to: combine the PetCO2 parameter score and the VE/VCO2 parameter score to generate a pulmonary arterial hypertension score; and compare the pulmonary arterial hypertension score to at least one predetermined threshold value for pulmonary arterial hypertension.

6. The gas exchange system of claim 5 , wherein the computing device being configured to combine the PetCO2 parameter score and the VE/VCO2 parameter score includes the computing device being configured to sum the PetCO2 parameter score and the VE/VCO2 parameter score.

7. The gas exchange system of claim 1 , wherein the computing device being configured to estimate a ventilatory threshold based on the plot includes the computing device being configured to: identify a middle portion of the plot based on comparing the multi-breath average rate of oxygen uptake of the multi-breath groups to an upper threshold value; fit a first line to the identified middle portion of the plot; identify an upper portion of the plot based on comparing the multi-breath average rate of oxygen uptake of the multi-breath groups to the upper threshold value; determine whether at least one multi-breath group in the identified upper portion has a minute ventilation value that exceeds the first line by at least a deviation threshold; and when determined that at least one multi-breath group in the identified upper portion has a minute ventilation value that exceeds the first line by at least the deviation threshold: identify a breath group from the at least one multi-breath group exceeding the first line by the deviation threshold having the lowest rate of oxygen uptake; fit a second line to the identified breath group and a breath group having a peak oxygen uptake value; determine an intersection point of the first line and the second line; and determine the ventilatory threshold as being the greater of the rate of oxygen uptake at the intersection point and the rate of oxygen uptake of the identified breath group.

8. The gas exchange system of claim 7 , wherein the computing device being configured to fit the first line to the identified middle portion includes the computing device being configured to use least squares fitting on the multi-breath groups of the middle portion of the plot.

9. The gas exchange system of claim 1 , wherein the computing device is further configured to guide the patient or a caregiver through performance of the submaximal gas exchange test.

10. The gas exchange system of claim 9 , wherein the computing device being configured to guide the patient or caregiver through the performance of the submaximal gas exchange test includes the computing device being configured to cause instructions to be displayed that instruct the patient to perform exercise at a target intensity level.

11. The gas exchange system of claim 1 , wherein at least one of the multi-breath groups is generated from eight sequentially occurring breaths.

12. A gas exchange system, the system comprising: a flow sensor configured to sense a respiratory flow of a patient; an analyzer configured to determine a composition of at least a portion of the respiratory flow of the patient; and a computing device including at least one processor and a computer-readable memory storage device storing instructions that, when executed by the at least one processor, cause the computing device to: receive breath-by-breath gas exchange measurements based on the respiratory flow of the patient captured by the flow sensor and the analyzer during a submaximal gas exchange test; plot minute ventilation against rate of oxygen uptake based on the gas exchange measurements; estimate a ventilatory threshold based on the plot; assign a PetCO2 parameter score based on a result of comparing an end-tidal partial pressure of carbon dioxide (PetCO2) at the estimated ventilatory threshold to at least one predetermined threshold value for PetCO2; assign a VE/VCO2 parameter score based on the result of comparing a minute ventilation/carbon dioxide production relationship (VE/VCO2) value at the ventilatory threshold to at least one predetermined threshold value for VE/VCO2; combine the PetCO2 parameter score and the VE/VCO2 parameter score to generate a pulmonary arterial hypertension score; estimate a likelihood that the patient has pulmonary arterial hypertension based on comparing the pulmonary arterial hypertension score to at least one predetermined threshold value for pulmonary arterial hypertension; and cause a user interface to be displayed, the user interface including a report that shows the estimated likelihood.

13. The gas exchange system of claim 12 , wherein the instructions that, when executed by the at least one processor, cause the computing device to combine the PetCO2 parameter score and the VE/VCO2 parameter score include instructions that, when executed by the at least one processor, cause the computing device to sum the PetCO2 parameter score and the VE/VCO2 parameter score.

14. The gas exchange system of claim 12 , wherein the instructions that, when executed by the at least one processor, cause the computing device to plot minute ventilation against rate of oxygen uptake include instructions that, when executed by the at least one processor, cause the computing device to: group the breath-by-breath gas exchange measurements into multi-breath groups, each multi-breath group containing measurements for a plurality of sequentially occurring breaths; generate a multi-breath average minute ventilation for each of the multi-breath groups by averaging minute ventilation values for each of the breaths therein; generate a multi-breath average rate of oxygen uptake for each of the multi-breath groups by averaging rate of oxygen uptake values for each of the breaths therein; and plot a data point for each of the multi-breath groups based on the multi-breath average minute ventilation and the multi-breath average rate of oxygen uptake.

15. The gas exchange system of claim 14 , wherein the instructions that, when executed by the at least one processor, cause the computing device to estimate a ventilatory threshold based on the plot include instructions that, when executed by the at least one processor, cause the computing device to: identify a middle portion of the plot based on comparing the multi-breath average rate of oxygen uptake of the multi-breath groups to an upper threshold value; fit a first line to the identified middle portion of the plot; identify an upper portion of the plot based on comparing the multi-breath average rate of oxygen uptake of the multi-breath groups to the upper threshold value; determine whether at least one multi-breath group in the identified upper portion has a minute ventilation value that exceeds the first line by at least a deviation threshold; and when determined that at least one multi-breath group in the identified upper portion has a minute ventilation value that exceeds the first line by at least the deviation threshold: identify a breath group from the at least one multi-breath group exceeding the first line by the deviation threshold having the lowest rate of oxygen uptake; fit a second line to the identified breath group and a breath group having a peak oxygen uptake value; determine an intersection point of the first line and the second line; and determine the ventilatory threshold as being the greater of the rate of oxygen uptake at the intersection point and the rate of oxygen uptake of the identified breath group.

16. The gas exchange system of claim 15 , wherein the instructions that, when executed by the at least one processor, cause the computing device to fit the first line to the identified middle portion include instructions that, when executed by the at least one processor, cause the computing device to use least squares fitting on the multi-breath groups of the middle portion of the plot.

17. The gas exchange system of claim 12 , wherein the instructions, when executed by the at least one processor, further cause the computing device to guide the patient or a caregiver through performance of the submaximal gas exchange test.

18. The gas exchange system of claim 17 , wherein the instructions that, when executed by the at least one processor, cause the computing device to guide the patient or caregiver through the performance of the submaximal gas exchange test include instructions that, when executed by the at least one processor, cause the computing device to cause instructions to be displayed that instruct the patient to perform exercise at a target intensity level.

19. A gas exchange system, the system comprising: a flow sensor configured to sense a respiratory flow of a patient; an analyzer configured to determine a composition of at least a portion of the respiratory flow of the patient; and a computing device including at least one processor and a computer-readable memory storage device storing instructions that, when executed by the at least one processor, cause the computing device to: receive breath-by-breath gas exchange measurements based on the respiratory flow of the patient captured by the flow sensor and the analyzer during a submaximal gas exchange test; estimate a ventilatory threshold based on the gas exchange measurements; calculate a PetCO2 parameter score based upon an end-tidal partial pressure of carbon dioxide (PetCO2) value at the ventilatory threshold; calculate a VE/VCO2 parameter score based on a minute ventilation/carbon dioxide production relationship (VE/VCO2) value at the ventilatory threshold; combine the PetCO2 parameter score and the VE/VCO2 parameter score to generate a pulmonary arterial hypertension score; estimate a likelihood that the patient has pulmonary arterial hypertension based on comparing the pulmonary arterial hypertension score to at least one predetermined threshold value for pulmonary arterial hypertension; and cause a user interface to be displayed, the user interface including a report that shows the estimated likelihood.

20. The gas exchange system of claim 19 , wherein the instructions, when executed by the at least one processor, further cause the computing device to alter the submaximal gas exchange test based on the estimated ventilatory threshold.